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Author Topic: Sony A7rii for photogrammetry and lenses  (Read 4092 times)

johnymal

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Sony A7rii for photogrammetry and lenses
« on: January 22, 2020, 09:21:58 AM »
Hello i recently bought a camera sony A7rii used for photogrammetry projects as an architect i want it only for building and facades. Now i own a sigma 24mm prime lense but i want to buy a wider like 14mm i think. As i read in another topic some of the members use zoom lens 10-20mm or 12-24mm are they right for phtogrammetry? Anny other suggestions?

remotesense

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Re: Sony A7rii for photogrammetry and lenses
« Reply #1 on: January 24, 2020, 01:08:44 AM »
FYI.  The Sony A7rii camera uses 5-axis image sensor stabilization.  This means your sensor is moving which is a bad thing for photogrammetry.  You'll have alignment errors within your data.  You should at least turn that option off if you use that camera, but its hard to say how rigid that mechanism really is.

One of the rules of photogrammetry is consistent photos between your images, to maintain good geometry for the reconstruction (alignment step).  Don't go with a zoom lens, stick to prime lens to eliminate the focal length movement between your photos.  Plus they are sharper, which helps support pixel matching producing accurate tie points to sub pixel levels.   Also, a wide angle lens will typically have a better base to height ratios, helping the software calculate your distance to that surface more accurately.

johnymal

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Re: Sony A7rii for photogrammetry and lenses
« Reply #2 on: January 24, 2020, 09:40:27 PM »
So cost is not a problem what camera and what lens do you suggest to have best quality of photos.
A canon SLR maybe? I do not want to go to medium format cameras because is very heavy..

wojtek

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Re: Sony A7rii for photogrammetry and lenses
« Reply #3 on: January 25, 2020, 03:15:28 PM »
FYI.  The Sony A7rii camera uses 5-axis image sensor stabilization.  This means your sensor is moving which is a bad thing for photogrammetry.  You'll have alignment errors within your data.  You should at least turn that option off if you use that camera, but its hard to say how rigid that mechanism really is.

One of the rules of photogrammetry is consistent photos between your images, to maintain good geometry for the reconstruction (alignment step).  Don't go with a zoom lens, stick to prime lens to eliminate the focal length movement between your photos.  Plus they are sharper, which helps support pixel matching producing accurate tie points to sub pixel levels.   Also, a wide angle lens will typically have a better base to height ratios, helping the software calculate your distance to that surface more accurately.

Your image stabilization point is a bit untrue, you should disable it only if you shoot from a tripod (because the camera tries to compensate for motion that is not there and degrades the image quality). If you are using it hand-held keeping it on is a must.

dcobra

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Re: Sony A7rii for photogrammetry and lenses
« Reply #4 on: January 29, 2020, 04:06:39 PM »
The Sony A7Rii does have sensor stabilization, which likely isn't good for photogrammetry as remotesense has pointed out.  The literature recommends turning all such features off.  I'm still waiting for a good paper to assess whether or not turning those features off is enough or if cameras with them should be avoided. 

I have been successfully using an A7Riii with a Zeiss Batis 18 mm lens with good results.  I have compared measurements taken from the models to third party measurements and note differences on the order of a few mm on models ~15 m in size.  I do wonder if I would see an improvement if I used a camera with a fixed sensor.

Something to consider is that the sensor does freely move around when the camera is turned off and it is only when the camera is powered on that it is rigidly held in place.  If you power the camera off during a shoot and then back on, the sensor may not be in the same position as previously.  I don't know what the precision of the sensor positioning system is.  I would suggest leaving the camera powered on during an entire shoot, and if that is not possible then possibly creating separate camera calibrations.

Hope that helps
Cody

dcobra

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Re: Sony A7rii for photogrammetry and lenses
« Reply #5 on: January 29, 2020, 04:20:53 PM »
Forgot to mention that in addition to the sensor moving freely when the camera is powered of, the lens also moves focus (to infinity I believe).  When powered off and then back on the focus moves back to the previous setting, unless the battery is removed then it resets to infinity, but again the precision is unknown.  It likely isn't the exact same position and my suspicion is that this creates an even greater error than a shift in sensor position.  I would recommend using a fully manual lens and taping the focus ring if possible.  If not consider my previous recommendation.

Mak11

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Re: Sony A7rii for photogrammetry and lenses
« Reply #6 on: January 29, 2020, 07:08:03 PM »
Unless some project requires sub-millimeter accuracy for some kind of measurement &/or the camera is used indoors in a studio setting on a tripod, stabilisation should always be ON.
A wide aperture is really important for better depth perception & reconstruction in photogrammetry. The narrower the aperture the more stable the camera has to be even on a tripod when outdoors if it's windy. Focal length should also stay the same  during the whole shooting.

Mak
« Last Edit: January 30, 2020, 12:53:08 PM by Mak11 »

James

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Re: Sony A7rii for photogrammetry and lenses
« Reply #7 on: January 30, 2020, 12:46:45 PM »
Found in this book:

https://books.google.ru/books?id=_f7oBQAAQBAJ&pg=PA188&lpg=PA188&dq=image+stabilisation+for+photogrammetry&source=bl&ots=TZ20SsaF5r&sig=YjHtTi2du7NqNvONGzl-1UN9BWQ&hl=en&sa=X&ved=0ahUKEwid8_nKlKPaAhXpCJoKHWCpAiUQ6AEIdTAH#v=onepage&q=image%20stabilisation&f=false

"Geometric changes due to Image stabilization can only be accommodated by use of Image-variant camera calibration"

Metashape does allow this but you have to enable it specifically and know what you're doing.

However i don't know how much difference it actually makes in practice, and you might still get superior results if the increase in image quality from a stabilised lens/sensor outweighs the drop in quality from having inconsistent lens/sensor geometry.

A wide aperture is really important for better depth perception & reconstruction in photogrammetry. The wider the aperture the more stable the camera has to be even on a tripod when outdoors if it's windy.

I think that's the wrong way around and you mean narrow aperture/large F number.

Mak11

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Re: Sony A7rii for photogrammetry and lenses
« Reply #8 on: January 30, 2020, 12:49:00 PM »
Yes that what I meant. Sorry for the typo.

Mak